Until recently, lead titanate (PZT) compounds were a favorite for use as piezoelectric materials in acoustic imaging transducers and similar devices requiring a high voltage sensitivity and electro-mechanical coupling coefficient, as well as low acoustic impedance and an ability to be formed-into arbitrary shapes. While proponents of PZT have touted its high sensitivity and relatively low cost, it has been discovered that the use of PZT compounds in connection with medical systems is inherently non-optimal because of the large acoustic impedance mismatch between the ceramic transducer and the mostly liquid body constituents. As a consequence of the mismatch in acoustic impedance, sensitivity bandwidth and acoustic wave acceptance angle can be sacrificed.
One alternative to the use of PZT compounds that has offered some solutions to this problem is the choice of the piezoelectric polymer, polyvinylidene fluoride (PVF.sub.2 or PVDF), as described by Swartz and Plummer in "Integrated Silicon-PVF.sub.2 Acoustic Transducers Arrays," IEEE Trans. Sonics Ultra Son., su-27, vol. ed-26, No. 12, pp. 1921-1931 (December, 1979), which is hereby incorporated by reference. This plastic material has a relatively low acoustic impedance which approximates that of water. Thus, PVF2 cuts down on the impedance mismatch associated with PZT compounds. The natural consequence of this improvement is increased bandwidth, sensitivity and acceptance angle, without the need of front matching layers.
In order for PVF.sub.2 films to be used as ferroelectric materials, they generally must be stretched mechanically as well as oriented electrically (poled) to convert the non-polar crystalline phase into a polar crystalline phase. Thus, prior artisans have used preformed films which were then epoxy glued to MOSFET transistor chips to produce an electric acoustic sensor. Swartz and Plummer describe such sensors in the above-mentioned article at pages 1921-22. This technique, however, was fraught with manufacturing and performance disabilities. It is understood that gluing preformed films onto transistor chips in small arrays is awkward. Additionally, the epoxy glue often presents a loss of sensitivity due to impedance mismatch and discontinuities in the glue layer.
Accordingly, there exists a need for a better manufacturing technique for preparing integrated silicon-polymer film ferroelectric arrays. There is also a need to produce low cost piezoelectric and pyroelectric sensors having better acoustic imaging of body tissues with higher voltage sensitivity and low impedance.